Note HCY' is very different from HSL and HCY. Chroma is perceptual, while Y' is gamma-corrected.

The main advantage of a HCY' picker is that it's just very intuitive. Here's the problem: different colors look brighter or darker at the same energy level. A 1 W/sr blue light will look noticeably darker than a 1 1W/sr yellow light. Ordinary pickers don't take this into account. The HCY' picker is designed to overcome this. Because the brightness is perceptually normalized, you don't have to worry about colors getting brighter or darker when you tweak them. Want a slightly cooler color? Slide the color towards blue. Want a warmer tone? Slide it towards red or orange. The computer takes care of the perceptual discrepancies in brightness between colors for you. It's great.

The screenshot is from MyPaint, which has the best damn color picker I've ever used.

Sounds like a cool feature, I'd be open to trying out a color picker like this, although I have noe issue with the current one, but different people have different minds and there are other uses for this type of color picker I guess, I guess the best part is that you never have to hue shift, just value.

support wrote:At the screen, the green at the border looks much brighter than the middle area

It's true, Mypaint's implementation is not perfect, also probably depends on screen calibration and gamut. It's still the best picker I've ever used. I suggest you download Mypaint and try it yourself. While Mypaint is generally too buggy to use in production, it should be fine for testing out the picker.

Something like this would indeed be very nice. I would love it. I don't know why they call it HCY though. This is actually how HSL works (Wikipedia*) – every (light) color eventually turns into white (or the other way, into black).

When I was a student I wrote a document about this whole issue. Below is a demonstration how HSL and HSV (aka HSB) sliders work.HSL sliders let you change hue, value and chroma independently.HSV (aka HSB) always changes value AND chroma together if you use the so called "saturation" or "brightness" sliders. (And this drives me nuts.)The picture below hopefully demonstrates what I just said.

Actually, I had an idea for a whole "3D"-Gamut HSL color slider but I don't think this is relevant here. But in case anybody (programmer/coder) would be interested in turning that concept into a real color picker that will dominate every color picker that the world has seen so far, then let me know.

I just had another idea that I wanted to share. I don't know if you are familiar with "Gamut masks" – maybe you read about it in Color and Light (written by James Gurney).So what about a color picker similar to MadMinstrels post, combined with the possibility to add a Gamut mask (similar to the Coolorus color picker) but with the difference of controlling the lightness (value) of every color with the lightness slider. – In other words a true HSL color picker with Gamut masks. That would be so freaking awesome. I made an example below:

I used 50% neutral gray for the mask with 60% transparency and 90% transparency for the border of the mask to stay visible at every lightness setting.A full chroma color ring around the color wheel helps to maintain a good orientation at very low or high lightness settings (slider).

Krita actually has color picker with the hue and saturation wheel, that helpful in cool-warm working. By the way, Krita has extended options of color schemes and profiles, you can choose between HSV, HSL, HSI, HSY' color models, moreover developers implemented it in layers blending.----------I think switching with button between the classical triangle color wheel and hue+saturation wheel or/and additional window with it would be good feature in PSS.